For simple cases, you could use this regexp:

while( $string =~ m/\bstart:(.+?)\bstop:/g ) {
    print "$1\n";
}
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...and to get the last match using that regexp, apply it like this:

my $lastmatch = ( $string =~ m/\bstart:(.+?)\bstop:/g )[ -1 ];
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<update>
...or for a pure regexp approach:

if( $string =~ m/\bstart:(?!.+?(?:\bstart:))(.+?)\bstop:/ ) {
    print "$1\n";
}
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...which only further punctuates the point I make in the next paragraph.
</update>

But that's not going to be very robust. What if your needs mature to the point that you can no longer guarantee that 'stop:' doesn't occur embedded within the portion of the string you're capturing? For example, if 'stop:' is wrapped in quotes, should it be treated as a delimiter, or as text? For a solution that will stand up to these sorts of complex strings, forget about hand crafting a masterful regular expression. The hard work has already been done, refined, debugged, tested, and proven. To take advantage of the work that's already been done, have a look at Text::Balanced.

use strict;
use warnings;

my $start = 'start:';
my $end = 'stop:';

$_ = join '', <DATA>;
@_ = m/\Q$start\E(.*?)\Q$end\E/gs;
print $_[-1];

__DATA__
something start: some text stop: other text here start: again somethin
+g stop: may be some text here start: and finally stop: and maybe some
+thing here
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pos = rindex($_, $start);
m/\Q$start\E(.*?)\Q$end\E/gs;
print $1;
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my $string = 'something start: some text stop: other text here start: 
+again something stop: may be some text here start: and finally stop: 
+and maybe something here';
my ($lastgroup) = $string =~ /.*start: (.*?) stop:/s;
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     You can use the following piece of code.

#!/usr/bin/perl -w
use strict;
my $filename = 'regex_data.txt';

open INPUTFILE , $filename or die "Cannot Open : $filename : $!";

while ( <INPUTFILE> )
{
   chomp $_;
   print "$2\n" if ( $_ =~ /^.*(start\:)(.*)(stop\:)/gi )
}
close INPUTFILE;
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      Assume that the file "regex_data.txt" contains your data. This code will print only the data between the last "start:" - "stop:" pair.

Hope this will help you !